Wednesday, June 03, 2009

Alignment vs. symmetry

A rant in less than 1000 words.

By Walt.

So my friend Mohawk is building (with assistance from me) himself a 29er frame to race on this year. Yesterday we were chatting about seatstays (I've ranted about what a pain it is to make them match up before on the blog) and he said "how asymmetrical do they have to be before you get speed wobbles?"

This brings up, for me, a rant topic: most people do not understand the difference between alignment and symmetry in bike frames. Nor do they understand the purpose of alignment very well.

Before I get started, I'll note that basically nobody understands speed wobbles (go read some of the hundred-page threads about them on various forums if you don't believe me), but asymmetry isn't generally thought to be a culprit.

Alignment consists of only a few things: BB shell, front and rear axles, and seat tube in plane (remember that the seat tube is actually perpendicular to this particular plane). This means the axles can't be offset to either side (if you drew a line through the center of both axles, it would pass through the center of the BB shell) and the axles and BB shell run parallel to each other (when the front wheel is straight, anyway). It also means that the seat tube joins the BB shell at exactly 90 degrees. You could spend some time describing this set of relationships more precisely, but that's pretty much the idea.

And that's it, really. No matter what shape the frame is, or how bent various tubes are, if those things all line up, your frame is in alignment. Here's an example of an aligned frame that is NOT symmetrical:

Other examples could include many full suspension bikes that use asymmetrical yokes to allow better tire/chainring clearance, Cannondale lefty forks, and your drivetrain (after all, unless you have a weird BMX, all the derailleurs, chainrings, and chain are on the right side).

The second part of this rant is about what alignment is, and how closely aligned a frame needs to be. I was inspired to post something after this thread showed up on the MTBR framebuilding forum. For the record, I always try to build a perfectly straight frame. But as in any real-world endeavor, geometrical perfection is only an abstract concept when the rubber hits the road - I'm sure if measured carefully, every frame I've ever built is crooked in one way or another. I'd bet any frame from any builder or company is measureably not straight as well - but it doesn't matter at all. I've ridden frames (before I started building them, as well as a few early ones I made for myself) that were *hilariously* out of alignment - we're talking 5-10mm, just look at the frame with one eye closed and you'll see it type stuff, not tiny amounts. They all rode pretty much fine, without any detectable pulling to one side or another, or weird behavior at any speed.

Remember too that the frame can be in great alignment and the bike can pull and ride weird - problems with headsets, hubs, forks, wheel dish and true, tires, saddles (try riding on a saddle with one bent rail sometime!), and even cable routing can cause bikes to pull to one side or another or behave oddly at speed.

So here's the bottom line: Bikes are *never* perfectly straight. You can go totally nuts trying to make them perfectly straight, but it won't matter much to how the bike rides unless something is *ridiculously* wacky. If front and rear wheels seem to sit straight in the frame/fork (remember, asymmetrical stays can make this a tough determination to make), and the bike doesn't pull to one side or exhibit any weird behavior, you should ride away into the sunset and never worry about alignment again.

If you take nothing else away from this, just remember: the proof is in the pudding. If your bike rides straight, it's as aligned as it needs to be.


Hua said...

Hi Walt,

Very informative post. Thanks for sharing.

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Thylacine said...

Awesome Walt, you're getting Spamed.

In Bicycle Guide magazine probably close to 20 years ago now there was the exact same kinda write-up on frame alignment. The gist is that the systems in place (sequential welding, alignment of tubes etc) when carried out correctly product a 'straight enough' frame the vast majority of the time. Any 'cold working' you do post welding should not be carried out like a dog chasing it's tail because you end up compromising the structure. The cold working is essentially just attempting to minimise the effect of any anomalies in the materials as they heat and contract during welding, as well as the influence of the mechanical interface of the tubes upon eachother.

What I got out of that article is essentially the same as I get out of discussion to this day - be as accurate as you can, mess with the frame as little as possible after it's welded, and only let it out the door if you can sleep at night.

Cheers for clearing up the difference for everyone between 'alignment' and 'symmetry', too. I hate to say it, but I certainly hope 'Alignment' is not the next buzzword after 'Lightweight' and 'Stiffness'.

Oh, and 'Vertically compliant, laterally stiff'. ;)

Bradley Wilson said...

"I must stress that you should not go over the top when 'dimensioning' frames. There are limits of sensibility and most frame builders know where they lie.... A good engineer is one who understands the relevance of each decision and its importance within the final requirement. The fact that our bodies are tolerant of many dimensional inaccuracies and only a few aspects of frame building are actually very important... these are: accurate tube mitering, complete and safe brazing (welding), and the back wheel perfectly following the front.... I believe in attempting to get it right is equally important not to forget the engineering reality of frame building; it is a process of fabrication worlds apart from the micrometers of machining."

-Tony Oliver (Old-School British Frame Builder)
From "Touring Bikes: A Practical Guide"